Your metabolic rate naturally continues to decline 5 percent each decade once you hit 40.3 Your resting metabolic rate (RMR) determines how many calories you just to survive. It is determined by a variety of chemical reactions inside your body. These reactions rely on your mitochondrial function and sodium-potassium pumps. Your mitochondria help to create energy, while your sodium-potassium pumps generate muscle contractions and nerve impulses.4,5,6,7,8 Both of these processes become less effective over the years and slow your metabolism.9,10 If you eat more than your RMR yet lead a sedentary life, you set yourself up for weight gain.

A 2014 study found that the type of fat we consume might make all the difference. Participants in the study were asked to eat 750 extra calories every day for seven weeks. Those having excess calories from saturated fats had activated cells that promoted fat storage in the belly and increased insulin resistance. However, individuals who had a high consumption of polyunsaturated fats found in fatty fish, nuts and seeds, gained less abdominal fat and were more likely to increase muscle mass instead.
As we age, we tend to become more insulin resistant which can put us at risk for type 2 diabetes. When you eat foods that break down into sugar, the pancreas pumps out insulin to escort the sugar out of your blood.  People with Insulin resistance don’t use insulin effectively so that cells have trouble absorbing sugar which causes a buildup of sugar and insulin in the blood.  Researchers still aren’t 100% in agreement as to why, but at the end of the day, people with insulin resistance gain weight, particularly around the middle.  And some research shows that lack of estrogen may cause insulin resistance.
People naturally lose muscle after 40, especially women after menopause. Because muscle burns more calories than fat, this can slow down your metabolism and make it harder to shake those stubborn pounds. Strength-training exercises -- lifting weights or doing body-weight exercises, like push-ups and squats -- at least twice a week can help you keep those muscles.
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